Bonding in MetalsSection 7.3

Objectives

When you complete this presentation, you will be able to … Model the valence electrons of metal

atoms.

Describe the arrangement of atoms in a metal.

Explain the importance of alloys.

Introduction

We are already familiar with metals

We know that metals are Tough

Ductile

Malleable

Conductive

These properties come from the way metal ions form bonds with one another.

Metallic Bonds

Metals are made up of closely packed cations rather than neutral atoms.

The valence electrons of metal atoms can best be modeled as a sea of electrons. These electrons are not associated with

any particular metal atom (they are not localized).

They are mobile and are free to drift from one part of the metal to another.

Metallic Bonds

Metallic bonds consist of the attraction of the free-floating valence electrons for the

positively charged metal ions.

These are the forces of attraction that hold metals together.

Metallic Properties

The “sea of electrons” model explains many of the properties of metals we are familiar with.

Conductivity: As we introduce an electron into the end of

a metal wire (for example)

electrons are free to move throughout the metal and will allow another electron to exit out the other end.

Metallic Properties

Ductility and Malleability: The sea of electrons isolates the metal ions

from one another.

As pressure is applied to the bulk metal, the metal ions are free to move and rearrange so that the metal can reform without damage.

This is unlike ionic crystals which are not isolated from each other and will fracture under pressure.

Crystalline Structure

The ions in many metals are arranged in a hexagonal pattern, much like fruits or vegetables in a produce section of a grocery store.

This arrangement is called a “hexagonal close-packed” (hcp) arrangement.

Every ion has twelve neighbors.

Metals with this arrangement include magnesium, Mg, zinc, Zn, and cadmium, Cd.

Crystalline Structure

Other metal ions may form a “body-centered cubic” (bcc) arrangement.

Every ion has eight neighbors.

This arrangement is used by sodium, Na, potassium, K, iron, Fe, chromium, Cr, and tungsten, W.

Crystalline Structure

Yet other metal ions may form a “face-centered cubic” (fcc) arrangement.

Every ion has twelve neighbors.

This arrangement is used by copper, Cu, silver, Ag, gold, Au, aluminum, Al, and lead, Pb.

Alloys

Few metals that we encounter are composed of just one type of metal.

Usually, we use mixtures of metals called alloys.

Alloys are mixtures of two or more metals. Brass is a mixture of copper and zinc.

Bronze is a mixture of copper and tin.

Alloys

Alloys are important because their properties often make them more useful than the pure metals. Sterling silver (92.5% Ag, 7.5% Cu) is

harder and more durable than pure silver.

Stainless steel (80.6% Fe, 18.0% Cr, 0.4% C, 1.0% Ni) is harder and less likely to corrode than pure iron.

Alloys

Alloys can form in many ways. If the ion replacing the pure metal ion is

about the same size, the new ion just fits into the crystal where the original ion would have been. This is called a substantial alloy.

Alloys

Alloys can form in many ways. If the ion is smaller than the pure metal ion

it is replacing, then the ion fits into the spaces between the metal ions. This is called an interstitial alloy.

Summary

Metals are made up of closely packed cations rather than neutral atoms.

The valence electrons of metal atoms can best be modeled as a sea of electrons.

The “sea of electrons” model explains many of the properties of metals we are familiar with. Conductivity

Malleability and Ductility

Summary

The ions in many metals are arranged patterns. Hexagonal-close packed

Body-centered cubic

Face-centered cubic

Alloys are mixtures of metals.

Alloys are important because their properties often make them more useful than the pure metals.